50% speedup with 400% more CPU power sounds like awful scalability to me.

Besides that LR already has full size JPEG renditions of your images in its cache. If your export-to-JPEG operation implied no further processing one can presume that LR used the rendering it already made when painfully importing your files, and just added any metadata that you specified for the export, instead of going back to the raw bits and re-created the exact same JPEG as it already had. Considering that, 14 minutes to export 1600 unprocessed JPEG files from EOS MkIII raws is *excruciatingly* slow on such a nice and fast i7 SSD box as yours: properly written software can get away with such a task in less than 3 minutes.